1. Field of the Invention
The present invention relates to a method for forming polycrystals of III-V compound semiconductive material, a semiconductor device utilizing said compound semiconductive material, and a printer and a display device utilizing said semiconductor device, and more particularly to the application thereof to a light-emitting device such as a light-emitting diode or an electroluminescence device.
2. Related Background Art
Applications of polycrystalline semiconductors are already known in the following fields. In the materials of group IV of the periodic table, mainly polycrystalline Si is employed in solar cells and thin film transistors. Among polycrystals of groups II-VI, Cd-based materials are employed in thin film transistors and photosensors, and are investigated for applications to solar cells. Also Zn-based materials are used in electroluminescence devices and fluorescent materials, and charcopyrite polycrystals such as CuInSe.sub.2 are being investigated for applications to solar cells.
Among the polycrystals of group III-V group, for example, Ga- and In-based materials were once investigated for application to solar cells, but have not been developed to the level of commercialization. The III-V polycrystals appeared in many reports concerning solar cells, but their light emitting characteristic has only been dealt with in a few reports. Salerno, J. P. et al. reported their electron beam luminescence in Cont. Rec. IEEE, vol. 15, p.1174-1178, but no description has been on the characteristic of light-emitting diode utilizing a p-n junction.
Conventionally, display devices utilizing light-emitting diodes have been prepared by forming such light-emitting diodes (LED) on a monocrystalline wafer, then cutting said LED's individually or in a group of several diodes, and adhering such diodes on a supporting substrate in the form of a lamp or a display device for characters or numerals. Also, a hybrid device including plural LED's as an LED display device of a large area has been array head, but the use of such large-area LED display has been limited because of the high cost.
In order to overcome the limitation in the display area of the LED display, the present inventors already proposed, in the EP Publication No. 284437A2, a selective nucleation method for forming III-V compound single crystal of a large area. This method consists of employing a substrate having a non-nucleation surface of a low nucleation density for the III-V compound crystals and a nucleation surface of an amorphous material positioned adjacent to said non-nucleation surface, and having a sufficiently small area for allowing crystal growth from a single nucleus and a nucleation density larger than that of said non-nucleation surface, and growing III-V compound single crystal from said single nucleus, eventually beyond the nucleation surface over the non-nucleation surface.
Also the present inventors disclosed, in the EP Publication No. 285358A2, formation of an LED on a non-monocrystalline substrate, by switching the crystal forming conditions in the course of the above-mentioned single crystal formation, thus forming a p-n junction area therein.
The LED array head as described in the Japanese Patent Application Laid-Open No. 60-48384, is prepared by adhering LED array of 1-2 cm each in length, prepared on a monocrystalline compound semiconductor substrate, in a linear arrangement on a supporting substrate.
Since such LED array head emits light principally in a direction perpendicular to the monocrystalline substrate, the supporting substrate 1902 is supported, as shown in FIG. 19, parallel to the surface of a photosensitive drum 1901. For this reason, considerable space has to be reserved around the photosensitive drum for installing the LED array head.
Japanese Patent Application Laid-Open No. 2-125765 proposes a configuration of arranging the monocrystalline substrate in such a manner that the light emitting direction becomes parallel to the supporting substrate and maintaining said supporting substrate in a direction perpendicular to the surface of the photosensitive drum. However, mounting the substrates requires arranging and adhering a plurality of small monocrystalline compound semiconductor substrate chips on the supporting substrate, and aligning the optical axis. Consequently a long time is required for mounting and there is often encountered unevenness in the pitch of the light-emitting elements and in the light intensity, at the junctions of such chips.
For overcoming such drawbacks, the present inventors proposed the semiconductor device utilizing the above-mentioned selective nucleation method.
The above-mentioned selective nucleation method enables to obtain a large-area III-V single crystal on a non-monocrystalline substrate, but there may sometimes be formed polycrystals or a void on the nucleation site. The preparation of LED elements on such substrate may show insufficient uniformity, since such polycrystals result in a lowered light emission intensity while such void gives rise to total lack of light emission. Besides, even the single crystal sometimes exhibits abnormal growth, assuming an oblong form with a strong anisotropy in the growth, thus hindering the device preparing process such as electrode formation. Furthermore, the selective nucleation method is associated with a contradicting property that the crystal occupancy rate is deteriorated under a crystal growth condition providing a high monocrystallinity, and vice versa.
Furthermore, since the above-mentioned light-emitting elements employing single crystal or polycrystals emit light in a direction perpendicular to the substrate, they require a considerable space around the photosensitive drum when they are employed as an LED array head in an electrophotographic printer. Also a planarization step, involved in the device preparing process, applies a physical impact onto the substrate surface, thus eventually causing peeling of the semiconductor crystal.